Polyquinazolone-based polymers are known to have particularly excellent chlorine resistance and alkali resistance and to show high resistance against various organic solvents and chemicals. Therefore, they can be formed into permselective membranes which are suitable for use in the membrane-separation of not only aqueous liquids but also non-aqueous liquids, such as ultrafiltration membranes and reverse osmosis membranes (see, for example, Japanese Patent Application No. 130214/79 corresponding to U.S. Patent Application Ser. No. 195,715, filed Oct. 9, 1980).
In producing permselective membranes from polymers, it has generally been necessary to dissolve the polymers in a water-soluble polar organic solvent to thereby prepare a dope. In many cases, aprotic basic polar organic solvents, such as N-methylpyrrolidone, dimethyl sulfoxide and dimethylformamide, have been used as such organic solvents.
To produce high molecular weight polyquinazolone-based polymers capable of being used to form films by polymerizing a bisoxazinone and a diamine as described hereinafter, the only method known is one wherein a sparingly water-soluble and protonic acidic polar organic solvent, such as cresol and chlorophenol, is used as a polymerization solvent (see, for example, Kogyo Kagaku Zashi, 73, 1239 (1970)). In this method, however, if a water-soluble polar organic solvent, such as N-methylpyrrolidone, is used as the polymerization solvent, the rate of polymerization is extremely low and only low molecular weight polymers having an inherent viscosity of about 0.3 or less are obtained even if the polymerization time is increased (see, for example, Kobunshi Ronbun Shu, 35, 369 (1978)). These low molecular weight polymers cannot be used to form films, or even if they can be formed into a film, the film thus-formed is very brittle and has low mechanical strength. Thus, permselective membranes which are practically usable cannot be prepared using such low molecular weight polymers.
Additionally, a method to prepare polyquinazolone from a diacylated aromatic diaminodicarboxylic acid and a diamine is known (see U.S. Pat. No. 3,444,136). This method, however, is different from the method of this invention described hereinafter wherein polyquinazolone-based polymers are prepared from a bisoxazinone and a diamine.
It is also known that on polymerization of a bisoxazinone and an aliphatic diamine in N-methylpyrrolidone, relatively high molecular weight polymers result. These polymers, however, are polyamides (see J. Polymer Sci., Polymer Chem. ed., 17, 1163 (1979)).
Therefore, in preparing permselective membranes made of polyquinazolone-based polymers, a complicated method has heretofore been employed which comprises polymerizing a bisoxazinone and a diamine in a sparingly water-soluble organic solvent, such as cresol, to provide a polymer solution, adding a non-solvent for the polymer to the polymer solution thus-formed to precipitate the polymer, isolating the precipitated polymer, and then dissolving the polymer in a water-soluble polar organic solvent, such as N-methylpyrrolidone, to prepare a dope. This complicated method is very disadvantageous in the commercial production of permselective membranes. In other words, if high molecular weight polyquinazolone-based polymers with film-forming capability can be prepared using a water-soluble and aprotic polar basic organic solvent, which is a dope solvent, as a polymerization solvent, the dope can be prepared using the polymerization solvent as it is, which is very advantageous in the commercial production of permselective membranes made of polyquinazolones.